When an object floats in any liquid, the weight of liquid displaced = the upthrust on the object.
example. ice is less dense than water. The RD is about 0.91
An ice cube of 1 cubic centimetre is put in water. It's weight = volume x RD = 0.91 g
So 0.91 g of water is displaced (pushed out of the way as the cube settles down in the water). So 91% is below the waterline and 9% above.
But the RD is relative. Because it depends what liquid you use as the reference that you measure other substances by.
If we lived on a liquid methane planet, we'd probably use that as the reference!
On this planet we usually use water as the reference and we have rigged the units so that 1 cc of water weighs 1 g; giving water itself a density of 1.
Water will float on a heavier liquid, such as mercury.
1/5 above the surface of the water so it's RD (= density = specific gravity) is 0.8
Let's say we have 1 cubic centimetre of the substance. Then its weight is 0.8 g.
Now it is placed in another liquid and it floats with just 1/20 above the surface.
That means it displaces 0.95 cc of that liquid and that amount of liquid must weigh 0.8 g (in order to hold the substance up).
So the density (specific gravity) of that substance (relative to water) must be = weight / volume = 0.8 / 0.95 = 0.84